1. Field of the Invention
The present invention relates to a light source apparatus using a laser light source and phosphors, and in particular, relates to a light source apparatus that emits visible light such as red, green, and blue light, and also to an image display apparatus using the light source apparatus.
2. Description of the Background Art
Today, a projector is widely used as an image display apparatus that projects various video images and the like onto a screen in an enlarged manner. The projector causes a spatial light modulation device, such as a digital micromirror device (DMD) or a liquid crystal display device, to collect the light emitted from a light source, and causes the light modulated with an image signal and emitted from the spatial light modulation device, to be displayed as a color image on the screen.
To obtain a bright large-screen video image in the projector, a high-pressure mercury lamp having a high brightness is used as a light source. The use, however, of a high-pressure mercury lamp as a light source has a problem that mercury is contained, and also causes a problem that a short lifetime of the light source complicates the maintenance.
To solve the problems, a proposal is made for a light source apparatus using a solid light source, such as light-emitting diodes (LED) or lasers, instead of a high-pressure mercury lamp, and also for a new image display apparatus using the light source apparatus.
A laser light source has a longer lifetime than a high-pressure mercury lamp. Further, the laser light emitted from the laser light source is coherent light, which therefore has highly directional properties and a high light-use efficiency. Furthermore, the laser light can ensure a wide color gamut due to its monochromaticity.
Due to its high coherence, however, the laser light causes speckle noise, which results in the deterioration of the image quality. In particular, laser light in the green and yellow wavelength ranges, which has a high luminosity factor for human eye, has a major problem that the image quality deteriorates due to speckle noise.
In contrast, in an LED light source, speckle noise caused by LED light as described above is not very problematic. The present circumstance, however, is that the LED light source has a large emission area and a low light density, which makes it difficult to achieve a high-brightness image display apparatus.
As a solid light source other than the LED light source and the laser light source, a proposal is made for a light source apparatus using LED light or laser light as excitation light to obtain the light emitted from phosphors, and also for an image display apparatus including the light source apparatus.
The use of laser light, which can be densely collected, as excitation light enables the light source apparatus using phosphors to obtain high-brightness light that only requires a small emission area. Further, if laser light is used as excitation light, the fluorescence per se obtained by wavelength conversion is incoherent light, which does not cause speckle noise.
As a conventional technique, a proposal is made for the structure of a light source apparatus where: a phosphor layer is formed on a wheel using a disk-shaped transparent base material; excitation light illuminates the phosphor layer; and fluorescence is obtained in space in the same direction as that of the excitation light source with respect to the surface on which the phosphor layer is formed.
The light source apparatus is intended for use in an image display apparatus, and therefore outputs light in the wavelength regions of three colors, namely red, green, and blue. Further, the light source apparatus has a reflection-type structure where fluorescence is obtained in space in the same direction as that of the excitation light source. This makes it possible to increase the conversion efficiency of excitation light to fluorescence. An LED light source, however, is used as a light source for obtaining red light, which makes it difficult to obtain high-brightness output light.
In addition, in a light source apparatus having a structure using a solid light source, to obtain high-brightness output light suitable for an image display apparatus, it is suitable to obtain green light and red light from fluorescence containing no speckle noise. A conventional method, however, has a problem with the obtaining of a high brightness from fluorescence.